The present invention relates to seamless backings for seamless coated abrasive articles. Additionally, this invention relates to methods of preparing seamless backings and seamless coated abrasive articles.
Backings or substrates used in coated abrasive articles are typically made of paper, polymeric materials, cloth, non-woven materials, vulcanized paper, or combinations of these materials. Many of these materials provide unacceptable backings for certain applications because they do not have sufficient strength, flexibility, or impact-resistance. In addition, some of these materials age too rapidly which is unacceptable. Furthermore, some of the materials are sensitive to liquids that are used as coolants and cutting fluids. Accordingly, early failure and poor functioning can occur in certain applications.
In a typical manufacturing process, a coated abrasive article is made by feeding a preformed backing in a continuous web form through a series of coating and curing steps wherein binder layers and abrasive particles are applied. The coated web is then converted into a desired construction, such as a sheet, disc, belt or the like. One useful construction of a coated abrasive article is an endless coated abrasive belt (i.e., a continuous loop of coated abrasive material). In order to form such an endless belt, the web form is typically cut into an elongate strip of a desired width and length. The ends of the elongate strip are then joined together to create a xe2x80x9cjointxe2x80x9d or a xe2x80x9csplicexe2x80x9d. Two types of splices are common in endless abrasive belts. These are the xe2x80x9clapxe2x80x9d splice and the xe2x80x9cbuttxe2x80x9d splice.
Although endless coated abrasive belts containing a splice in the backing are widely used in industry today, these products suffer from some disadvantages which can be attributed to the splice. For example, the splice is generally thicker than the rest of the coated abrasive belt, even though the methods of splicing generally used involve attempts to minimize this variation in the thickness along the length of the belt. This can lead to a region on the workpiece with a xe2x80x9ccoarserxe2x80x9d surface finish than the remainder of the workpiece, which is highly undesirable, especially in high precision grinding applications. For example, wood with areas having a coarser surface finish will stain darker than the remainder of the wood. Also, the splice can be the weakest area or link in the coated abrasive belt. In extreme cases the splice may break prematurely before full utilization of the coated abrasive belt, which leads not only to waste, but potential hazard. Belts have therefore often been made with laminated liners or backings to give added strength and support. Such belts can be relatively expensive and, under certain conditions, can be subject to separation of the laminated layers. In addition, abrading machines that utilize a coated abrasive belt may have difficulty in properly tracking and aligning the belt because the splice creates a discontinuity in the coated abrasive belt. Furthermore, the spliced area can be undesirably more stiff than the remainder of the belt, and belts including such a splice may put undesirable xe2x80x9cchatterxe2x80x9d marks on the workpiece. Finally, the splice in the belt backing adds considerable expense in the manufacturing process of coated abrasive belts.
There are known processes for producing seamless abrasive belts. For example, Ball (U.S. Pat. No. 2,404,207) discloses belts produced by a method that utilizes a carrier belt that is rotated around support rolls. A comb removes a carded membrane from a stripper roll to thereby deposit the carded membrane upon the rotating carrier belt. Accordingly, layers of carded membrane are incrementally deposited around a peripheral surface of the carrier belt as the carrier belt is rotated around the support rolls. The carded membrane can be comprised of fibrous materials such that layers of fibrous materials form a web about the carrier belt. A pressure roll is used to compact the web and impregnate the web with an adhesive binder material. Abrasive particles can also be distributed upon the carrier belt through two different control hoppers.
PCT International Publication No. WO 93/12911, published Jul. 8, 1993, discloses coated abrasives using fiber reinforced polymeric backings. In producing the backing, the fibers are engulfed by a polymer and the polymer is then solidified or cured, depending on the polymer""s chemistry. Abrasive particles are then adhered to the backing by a subsequent resin coating applied to the backing (sometimes referred to as a xe2x80x9cmakexe2x80x9d coating), typically a resole phenolic resin. The procedures for making the fiber reinforced backings are essentially batch procedures.
U.S. Pat. No. 5,681,612 (Benedict et al.) reports methods for preparing endless, flexible, seamless abrasive backings containing an organic binder material and a fibrous material embedded therein. The preferred method of forming endless, seamless abrasive backings in Benedict et al. is a batch process. The circumference of the belt. The backing binder precursor is coated onto the periphery of the drum and is solidified by exposure to an energy source (e.g., thermal or radiation energy). Before the backing can be removed from the drum, the binder precursor must be sufficiently cured or polymerized so that the binder precursor retains its shape (i.e., does not flow substantially) when removed from the drum.
In Benedict et al., preferred organic binder materials are thermosetting resins such as epoxy resins, urethane resins, polyester resins, or flexible phenolic resins. The most preferred resins are epoxy resins and urethane resins, at least in part because they exhibit acceptable cure rate, flexibility, thermal stability, strength and water resistance. Although these binder materials exhibit acceptable cure rates for thermally cured systems, they typically require a thermal cure on the drum for at least 20 minutes. Since the method of making endless, seamless abrasive backings is a batch process, the production rate is proportional to the time it takes to cure the binder precursor on the drum. For this reason, it is desirable to provide faster method of producing endless, seamless abrasive backings. In addition, it is desirable to provide a backing binder which has improved performance, for example, a decreased sensitivity to water and increased thermal resistance.
The present invention provides seamless backings for seamless coated abrasive articles (i.e., coated abrasive belts or loops) which have improved properties over known seamless backings. For example, the backing binder of a seamless backing of the present invention is more resistant to water (i.e., a lower water absorption) than conventional backing binders. In addition, the backing binder can be cured very quickly by exposure to radiation energy (e.g., ultraviolet light) allowing seamless backings of the present invention to be rapidly manufactured.
In one aspect, the present invention provides a seamless backing in the form of a belt (i.e., having a length, a width, a first and second generally parallel side edges, a first major exterior surface, and a second major interior surface). The seamless backing includes a backing binder comprising an interpenetrating polymer network formed by the polymerization of a backing binder precursor. The backing binder precursor includes:
(i) an aromatic polyisocyanate prepolymer;
(ii) a polyol curative;
(iii) an acrylated urethane; and
(iv) at least one polymerization agent.
The seamless backing further includes at least one fibrous reinforcing material which is engulfed within the backing binder. The term xe2x80x9cseamlessxe2x80x9d means that the backing has a substantially uniform thickness or caliper throughout. That is, it is free from thickened areas resulting from distinct splices or joints. This does not preclude, however, splices and/or gaps in a fibrous reinforcing material embedded within the backing. The term xe2x80x9cengulfedxe2x80x9d means that the fibrous reinforcing material is essentially completely encapsulated or embedded within the backing binder, so that there may be a very minor percentage of fibrous reinforcing material present at an outer surface of the backing.
The backing binder is formed by the polymerization of the backing binder precursor. The backing binder precursor comprises an aromatic polyisocyanate prepolymer, a polyol curative, an acrylated urethane, and at least one polymerization agent. As used herein xe2x80x9cpolyisocyanate prepolymerxe2x80x9d is a material that is intermediate between a monomer and a final polymer. A polyisocyanate prepolymer is the reaction product of a monomeric or polymeric isocyanate with itself or with other isocyanate reactive materials such that the polyisocyanate prepolymer has, on average, more than one unreacted isocyanate group per molecule. Isocyanate reactive materials include active hydrogen compounds, for example, polyols, polyamines, amine terminated polyols, and water.
The backing binder precursor cures via two distinct polymerization reactions (i.e., a dual cure mechanism) which occur simultaneously. A first polymerization reaction is an addition polymerization of the polyisocyanate with the polyol. Together, the polyisocyanate prepolymer and the polyol curative form a polyurethane precursor, that is, a composition which is capable of curing to form a polyurethane polymer. A second polymerization reaction is a free radical polymerization of the acrylate groups of the acrylated urethane to form a crosslinked acrylate.
The polyurethane and the crosslinked acrylate form an interpenetrating polymer network. As used herein xe2x80x9cinterpenetrating polymer networkxe2x80x9d or xe2x80x9cinterpenetrating networkxe2x80x9d means a mixture of two or more distinct polymers which are held together by permanent entanglements. The polymers may also be held together by some covalent bonding.
The backing binder precursor polymerizes to form an elastomeric binder. As used herein xe2x80x9celastomericxe2x80x9d means that the binder is flexible and has an elongation at break of at least 50%.
Preferred aromatic polyisocyanate prepolymers are based on 4,4xe2x80x2-diphenylmethane diisocyanate (MDI) and have a functionality ranging from about 2 to 3. As used herein xe2x80x9cbased onxe2x80x9d means that the polyisocyanate prepolymer uses the designated isocyanate as the isocyanate starting material from which the polyisocyanate prepolymer is formed. For example, xe2x80x9cbased on MDIxe2x80x9d means that the polyisocyanate prepolymer uses MDI as the isocyanate starting material of the polyisocyanate prepolymer.
Preferred polyol curatives are saturated polyether diols having the molecular formula HO[(CH2)4O]nH.
A preferred acrylated urethane is formed by reacting 2-hydroxyethyl acrylate with an aromatic polyisocyanate prepolymer based on MDI. Preferably, the acrylated urethane has less than about 0.01% by wt. residual urethane catalyst.
The backing binder precursor further comprises at least one polymerization agent. As used herein a xe2x80x9cpolymerization agentxe2x80x9d is a material which initiates and/or catalyzes a polymerization (i.e., curing) of the backing binder precursor. Preferred polymerization agents are urethene catalysts and free radical initiators. Preferred urethane catalysts are radiation activated urethane catalysts such as those described in U.S. Pat. Nos. 4,740,577 (DeVoe et al.) and 5,091,439 (Berner et al.).
In another aspect, the present invention provides a seamless coated abrasive article made from a seamless backing of the present invention. A preferred seamless coated abrasive article of the present invention includes a seamless backing of the present invention having an abrasive coating adhered to the exterior major surface thereof. The abrasive coating comprises a plurality of abrasive particles adhered to the seamless backing by a coating (i.e., a make coat) or multiple of coatings (i.e., a make coat and a size coat). Preferred make and size coatings comprise phenolic resins, more preferably resole phenolic resins. Optionally, a supersize coating may be applied over the size coat to provide a specific property such as antiloading.
In another aspect, the present invention provides a method for preparing a coated abrasive backing; the method including the steps of:
(a) providing a support drum having a peripheral surface;
(b) applying at least one fibrous reinforcing material over the peripheral surface of the drum;
(c) applying a backing binder precursor over the fibrous reinforcing material in sufficient amount to engulf the fibrous reinforcing material, the backing binder precursor comprising a mixture of:
(i) an aromatic polyisocyanate prepolymer;
(ii) a polyol curative;
(iii) an acrylated urethane; and
(iv) at least one polymerization agent;
(d) exposing the backing binder precursor to radiation energy to polymerize the backing binder precursor thereby forming an interpenetrating polymer network; and
(e) removing the seamless backing from the support drum.
In a preferred method, the peripheral surface of the support drum is first wrapped with a nonwoven mat, for example, a spunbond polyamide mat. Following application of the nonwoven mat, a continuous fibrous strand is wrapped in helical fashion around the drum over the nonwoven mat using a level winder. During the winding process, the backing binder precursor is applied over the nonwoven mat and continuous fibrous strand. The backing binder precursor engulfs the fibrous reinforcing materials.
The backing binder precursor is preferably supplied to the drum via a xe2x80x9cmeter and mixxe2x80x9d apparatus. The meter and mix apparatus consists of two separate vessels (i.e., vessel xe2x80x9cAxe2x80x9d and vessel xe2x80x9cBxe2x80x9d) with separate supply lines and pumping means. The separate supply lines feed a motionless mixer which functions to mix the material from vessel A with the material from vessel B. Vessel A holds the aromatic polyisocyanate prepolymer, and any desired optional ingredients (e.g., a polymerization agent or filler). Vessel B holds a mixture comprising a polyol curative, an acrylated urethane, and any desired optional ingredients (e.g., a polymerization agent). During the preferred method, the materials from vessel A and vessel B are mixed by the meter and mix system to form the backing binder precursor which is then applied over the fibrous reinforcing materials which are wrapped around the support drum. During application of the backing binder precursor, the coating head traverses the peripheral surface of the support drum following the level winder.
In a preferred embodiment, the acrylated urethane has less than about 0.01% by wt. residual urethane catalyst. In this way, the rate of the polymerization of the polyisocyanate prepolymer with the polyol is minimized until after the backing binder precursor is coated onto the fibrous reinforcing material.
Polymerization of the backing binder precursor may be accelerated by exposure of the backing binder precursor to a source of radiation energy. Preferably, the radiation energy is ultra violet light, or a combination thereof. In a preferred embodiment, the polymerization agent of the backing binder precursor comprises a free radical photoinitiator and a radiation activated urethane catalyst, which are both activated by ultraviolet light.